When collimated (parallel) radiation is incident upon the input of a lens system, radiation exiting from the output end will show one of three characteristics: (1) it will converge to a real point focus outside the lens system, (2) it will appear to diverge from a virtual point focus within the lens system, or (3) it will emerge as collimated radiation that may differ in some characteristics from the incident collimated radiation. In cases 1 and 2, the paraxial imaging properties of the lens system can be modeled accurately by a characteristic focal length and a set of fixed principal surfaces. Such lens systems are sometimes referred to as focusing or focal lenses, however they are usually referred to simply as lenses. In case 3, a single finite focal length cannot model the paraxial characteristics of the lens system; in effect, the focal length is infinite, with the output focal point an infinite distance behind the lens, and the associated principal surface an infinite distance in front of the lens. Such lens systems are referred to as "afocal," or without focal length. They are referred to as "afocal lenses," following the common practice of using "lens" to refer to both single element and multi-element lens systems.
A simple afocal lens can be made up of two focusing lenses set up so that the rear focal point of the first lens coincides with the front focal point of the second lens. There are two general classes of simple afocal lenses, one in which both focusing lenses are positive, and the other in which one of the two is negative. Afocal lenses containing two positive lenses were first described by Johannes Kepler and are called Keplerian. Afocal lenses containing a negative lens are called Galilean. Generally, afocal lenses contain at least two powered surfaces, with the simplest model for an afocal lens consisting of two thin lenses.
The combination of a first lens having a positive refractive power (the "first" lens being the lens nearest the object) and a second lens having a negative refractive power is a Galilean configuration. The combination with the first lens having a negative refractive power and the second lens having a positive refractive power is referred to as an inverse Galilean configuration.
Afocal attachments to lens systems can compress or expand the scale or shape of an image in one axis. Such devices are called "anamorphosers," or "anamorphic afocal attachments." One class of anamorphoser is the cylindrical galilean telescope. The keplerian form is seldom if ever used, since a cylindrical keplerian telescope would introduce image inversion in one direction. Anamorphic compression can also be obtained using two prisms.
There are increasing requirements for illumination systems that can provide anamorphic beam shaping. One such requirement is in the field of photolithography in which illumination of a non-symmetrical area with collimated energy is needed. Another such requirement is in the field of laser beam shaping in which, for example, there is a need to shape the elliptical output from a semiconductor diode laser into a desired circular output shape. Another requirement is to provide beam shaping in those spectral regions in which there are no refractive materials appropriate for the energy in those spectral regions, for example x-ray applications.
A current method of producing an illuminated area having a desired shape is shown in FIG. 1 in which a collimated beam 102 having a power P.sub.IN illuminates a mask 104 with an aperture 106 having the shape of the desired illuminated area 100. The illuminated area 100 has a power P.sub.OUT that is less than P.sub.IN and P.sub.OUT depends upon the size of the aperture 106 relative to the size of the input collimated beam 102. This method is satisfactory if efficiency is not a problem or concern in the system. The efficiency .eta.=P.sub.OUT /P.sub.IN where P.sub.OUT is the power in the output beam 100 and P.sub.IN is the power in the input beam 102. As can be appreciated the efficiency can be very low.
Another method of providing a scaled or shaped beam has been to use prisms or cylindrical lenses to provide anamorphic scaling of input beams. Such an anamorphic system 200 is shown in FIG. 2. The anamorphic system 200 has a positive cylindrical lens element 202 and a negative cylindrical lens element 204 to shape an incoming beam 206 into an anamorphic output beam 208. The efficiency .eta. of such a system is P.sub.OUT /P.sub.IN where P.sub.OUT is the power in the output beam 208 and P.sub.IN is the power in the input beam 206. Assuming there is no transmission loss in the lens elements, the efficiency .theta..apprxeq.1. However, the lens option is limited to spectral regions for which there are refractive materials available to construct cylindrical lenses or prisms. In addition, if the input beam is broad band, the lens assembly introduces chromatic aberration.
FIG. 3 shows a mirror equivalent 300 to the anamorphic system 200 shown in FIG. 2. An input beam 302 is incident on Mirror, 304, and then on Mirror.sub.2 306. To obtain anamorphic shaping, a surface of Mirror.sub.1 304 and Mirror.sub.2 306 are cylindrical. The output beam 308 is shown rotated 90.degree. for illustrative purposes and indicates anamorphic scaling of the output beam 308. When the system is configured having a common axis as shown in FIG. 3, the output beam 308 has the central region 310 obscured because of Mirror.sub.1 304. The obscuration 310 is the shadow of Mirror.sub.1 304.
The prior art systems discussed above either have low efficiency, exhibit optical aberrations or have an obscured output beam.
Accordingly, there is a need for an apparatus and method for producing an afocal, uniformly illuminated area having a desired shape with high transmission efficiency and minimum optical aberrations.